Signal measurement systems and methods

ABSTRACT

A measurement system and method for conducting measurements on a device-under-test (DUT). The system includes, in one embodiment, a passive impedance controlling tuner, and a signal transmission line, the tuner including a signal transmission line segment as at least part of the signal transmission line. A signal coupling device is coupled in a non-contacting relationship to the signal transmission line between a signal port of the DUT and the tuner for sampling signals propagating between the passive impedance controlling tuner and the DUT to allow measurement of an actual impedance presented to the DUT with the DUT in place in the measurement system during measurement of DUT characteristics. Measurement system equipment receives response signals from the signal coupler. The measurement system is configured to conduct measurement of DUT characteristics without pre-characterizing the impedance controlling tuner.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 13/292,075,filed Nov. 8, 2011, in turn a continuation of application Ser. No.12/477,851, filed Jun. 3, 2009, which is a division of application Ser.No. 11/419,691, filed May 22, 2006, which claims the benefit under 35USC Section 119(e) of provisional application Ser. No. 60/689,405 filedJun. 10, 2005, the entire contents of which applications areincorporated herein by this reference.

BACKGROUND

A Radio Frequency (RF) measurement system is one that measures somethingabout a Device Under Test (DUT) by sampling and measuring signalsapplied to and coming from the DUT. A vector measurement system willmeasure both magnitude and phase information, while a scalar measurementsystem will measure magnitude only.

A “signal analyzer” measures properties of a signal relative to itself,such as magnitude or phase vs. frequency or time. A “network analyzer”measures properties of a signal at a specific reference plane, so thatmany of the measured signal properties can be related to characteristicsof the DUT itself.

In this document, a “tuner system” will refer to a RF measurement systemwhich uses some kind of tuner or tuners to control impedance seen by theDUT.

An “automated tuner” may be computer controlled; a “manual tuner” iscontrolled manually by the user.

A “passive tuner” controls the impedance seen by the DUT by changinghardware settings which affect the passive reflection. The maximumreflection is limited by the physical hardware and losses between thetuner and the DUT.

An “active tuner” controls the impedance seen by the DUT by feeding asignal back into the DUT with a specific magnitude and phase relative tothe signal from the DUT. The impedance seen by the DUT will result froma combination of the passive reflections in the circuit and the “active”signal fed back to the DUT. In principle, the maximum effectivereflection can be up to or even greater than unity. In practice, this islimited by the amount of power generated by the measurement system thatcan be fed back to the DUT to synthesize that impedance.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the disclosure will readily be appreciated bypersons skilled in the art from the following detailed description whenread in conjunction with the drawing wherein:

FIG. 1 is a simplified block diagram of an exemplary embodiment of ameasurement system employing couplers.

FIG. 2 depicts an alternate embodiment of a passive measurement systemin which couplers are located inside the tuners

FIG. 3 illustrates a cut-away isometric view of a portion of anexemplary arrangement of a probe coupler installed in a slab line typeof tuner.

FIG. 4A illustrates a probe end of an exemplary coaxial probe set inisometric view.

FIG. 4B is a diagrammatic view illustrating a transverse position of theprobes relative to the tuner center conductor. FIG. 4C is a diagrammaticview illustrating a serial position of the probes along the centerconductor.

FIGS. 5A and 5B illustrate respectively an embodiment of a probe bracketand a probe bushing.

FIG. 6 illustrates an exemplary implementation of a probe coupler in amicrostrip-style solid state tuner.

FIG. 7 is a simplified block diagram of an exemplary load pullmeasurement system capable of measuring output power, transducer gain,and efficiency.

FIG. 8 is a simplified block diagram of an exemplary load pullmeasurement system, similar to that of FIG. 7, but with signal couplersadded between the tuners and the DUT.

FIGS. 9A-9B illustrate an embodiment of a probe coupler mounted in aseparate housing structure.

FIGS. 10A-10B illustrate another embodiment of a probe coupler mountedin a separate housing structure.

FIG. 11 is a flow diagram of an exemplary embodiment of a method forperforming a measurement.

DETAILED DESCRIPTION

In the following detailed description and in the several figures of thedrawing, like elements are identified with like reference numerals. Thefigures are not to scale, and relative feature sizes may be exaggeratedfor illustrative purposes.

An exemplary embodiment of a passive measurement system 50 employingcouplers is depicted in the simplified block diagram of FIG. 1. Thesystem 50 is for conducting measurements on a DUT 10, which may be, forexample and without limitation, power transistors, power FETs or otherpower amplifying devices, small-signal low noise transistors (e.g. FET,HEMT, PHEMT) or other amplifying devices, frequency translating devicessuch as frequency multipliers, and three-terminal devices such asmixers. An input coupler 60 is connected between an input port of theDUT and an input tuner 52. An output coupler 70 is connected between anoutput port of the DUT and an output tuner 54. The tuner systems may be,for example, a model MT980F13 tuner system for power and noisemeasurement from Maury Microwave using MT982E tuners. Measurementequipment 80 provides the excitation signals to the input tuner 52, andreceives response signals from the couplers 60, 70 and the output tuner54.

In the embodiment of FIG. 1, the couplers are located on the DUT side ofthe respective tuners. FIG. 2 depicts an alternate embodiment of apassive measurement system 100 in which the couplers 60, 70 are locatedinside the tuners 102, 104.

The couplers 60, 70 are typically mounted in a fixed position, and thusare not movable relative to the DUT.

FIG. 3 illustrates a cut-away isometric view of a portion of anexemplary arrangement of a probe coupler set in a slab line type oftuner 120, such as a model MT982E marketed by Maury Microwave, Ontario,Calif. The tuner includes a slabline transmission line structure 130,which comprises a center conductor 132 and opposed ground planestructures 134, 136. A panel or support structure 140 includes an endpanel 140A, attached to ends of the respective ground plane structures134, 136. A distal end of the center conductor 132 is passed through anopening in the end plate to a DUT connector. The apparatus furtherincludes a probe coupler set 150 mounted through an opening 154 in abracket 152, affixed across the ground plane structures 134, 136. A setscrew 152A fixes the position of the probes in the bracket, which mayinclude a probe bushing described below.

The probe set 150 includes a pair of coaxial probes 150A, 150B. FIG. 4Aillustrates a probe end of the probe set 150 in isometric view. Probe150A is a capacitive probe, including a center conductor 150A-1, adielectric sheath 150A-2, and an outer conductive shield 150A-3. Anexposed end 150A-11 of the center conductor extends beyond the outershield, and is positioned closely adjacent to, but not in contact with,the center conductor 132 of the slabline structure. The second coaxialprobe 150B is an inductive probe, and similarly includes a centerconductor 150B-1, a dielectric sheath 150B-2 and a conductive outershield 150B-3. For the current probe, the exposed end of the centerconductor is bent around and the tip brought into contact with the outershield 150B-3 to form an inductive loop. The tip end may be attached,e.g. by soldering, to the outer shield. The spacing between the probesand the center conductor may be adjusted or selected to achieve thedesired coupling value. Positioning the probes closer to the centerconductor results in tighter coupling (e.g. more signal coupled out),and farther from the center conductor results in looser coupling (e.g.less signal coupled out). This may easily be set empirically whilemonitoring the display of a network analyzer connected from the centerconductor input to the probe connector output, with the other end of thecenter conductor terminated in 50 Ohms.

The probe 150 in an exemplary embodiment, for a 7 mm tuner with a centerconductor diameter of 0.1197 inch, may be fabricated of 0.085 inchdiameter semi-rigid coaxial cable. In one exemplary embodiment, thecenter conductor protruded from the shield by about one diameter of thecable; this dimension can also be adjusted or varied to affect thecoupling value and the frequency response. One exemplary spacing range,for the spacing of the probe tips from the tuner center conductor, is0.050 inch to 0.150 inch. In an exemplary embodiment, the probes 150A,150B are mounted in the bracket 152 such that the center conductors ofeach probe are in the same plane perpendicular to the axis of the centerconductor 132. This arrangement is illustrated in FIG. 4C. In anotherembodiment, the probes are mounted in the bracket 152 such that thecenter conductors of each probe are in a plane coincident with thecenter conductor, as illustrated in FIG. 4B. In each embodiment, theloop of the inductive probe should preferably be oriented so that it isparallel to the center conductor of the tuner.

In an exemplary embodiment, the distal ends of the probe cables arebrought out to connectors mounted in side plate 140B for connection to,e.g. a measurement equipment such as measurement equipment 80 (FIG. 2).

The bracket 152 is fixed in position relative to the slabline structure,e.g. by threaded fasteners 152B. The bracket 152 and a probe bushing 156are illustrated in FIGS. 5A and 5B. In an exemplary embodiment, theprobe set 150 is held in a slot 156 formed in the bushing 156. Thebushing may be rotated within the hole 154 to position the probes ateither the position shown in FIG. 4B or the position shown in FIG. 4C,with set screws 156B securing the bushing at the selected location inthe bracket hole.

Another exemplary embodiment of an installation of a probe coupler in atuner is the tuner 200 depicted in the isometric cut-away view of FIG.6. The exemplary tuner here is a microstrip-style solid state tuner,such as the model NP5 or LP2 model tuner marketed by Maury Microwave. Amicrostrip transmission line 210 is formed by dielectric substrate 212disposed on a ground plane 202 of the housing, and a center conductortrace 214 on the top surface of the substrate 212. The distal end of theconductor trace 214 is connected to a DUT connector (not shown in FIG.5) on the outside of panel 208A.

A probe mounting bracket 220 is supported above the microstriptransmission line by housing brackets 204 and 206. A probe set 150 ismounted through opening 226 in the bracket, and includes a pair ofcoaxial probes as with the embodiment of FIG. 4A. A set screw 222 may beused to fix the probe set in position in the opening 226. Threadedfasteners 224 may fix the bracket 220 to the brackets 204, 206. Thedistal ends of the coaxial cables from the probes are connected to panelmount connectors 156, 158 on panel 208B.

As with the embodiment of FIG. 4A, the probes 150A, 150B of theembodiment of FIG. 6 are positioned in the mounting bracket 220 suchthat the center conductors of the probes are disposed in a planeperpendicular to the longitudinal extent of the center conductor trace214. In another embodiment, the probe center conductors may bepositioned in series along the trace 214, analogous to the orientationdepicted in FIG. 4B. The inductive probe is positioned preferably sothat the loop is oriented parallel to the center conductor trace 214.

FIG. 7 is a simplified block diagram of an exemplary load pullmeasurement system 300 capable of measuring output power, transducergain, and efficiency. An RF source 302 supplies an RF drive signalthrough a bias Tee 304, an input tuner 306, and an input fixture 308 toan input port for the DUT 10. The response of the DUT at an output portis passed through the output fixture 310, the output tuner 312 and thebias Tee 314 to the power meter 320. A bias system 322 applies a bias tothe bias Tees 304, 314. Non-RF elements of the system have been omitted,e.g. the computer that controls and automates the measurements is notshown in FIG. 7. One example of a commercially available tuner suitablefor use as tuner 306 and for tuner 312 is the model MT982E tuneravailable from Maury Microwave. The fixtures 308, 310 are normally usedto connect the DUT (however it is packaged) to a coaxial referenceplane. The input and output fixtures are usually supplied by the user tofit the particular device to be measured. Wafer probes, sold by CascadeMicrotech are very common examples. Another example is a fixture forsmall packaged devices, the model MT950, marketed by Maury Microwave;this exemplary fixture contains both the input and output.

There are many variations possible for load pull measurement systems.For example, the input and output fixtures 308, 310 are both optional.Additional tuners can be added for pre-matching or harmonic tuning.Either the source or load tuner could be omitted. A variety of otherequipment could be added, including but not limited to additional powermeters, spectrum analyzers, vector signal analyzers, RF switches, vectornetwork analyzers, large signal analyzers, or a noise figure meter.Different configurations can allow different sets of measuredparameters, measurement speed differences, or different measurementaccuracy, for example. The load pull system is often used to measurelarge signal parameters of power devices, which may be non-linear.However load pull in general would also include other applications thatuse impedance tuning, such as small signal, noise parametercharacterization, or DC-IV characterization with control of thetermination impedances.

FIG. 8 is a simplified block diagram of an embodiment of an exemplaryload pull measurement system, similar to that of FIG. 7, but with signalcouplers 360, 362 added between the tuners and the DUT 10. A signalmeasurement instrument 370 receives the signals from the signal couplerprobe elements, e.g. the coaxial probes of probe set 150. The samevariations mentioned about the system of FIG. 7 would apply here also.The signal measuring instrument 370 may be selected based on the desiredmeasured data, but examples include but are not limited to a vectornetwork analyzer (VNA) or a large signal network analyzer (LSNA).

FIGS. 9A-9B illustrate an embodiment of a probe coupler mounted in aseparate housing structure. The coupler 400 includes an exemplaryembodiment of a separate housing structure for signal probes usingslab-line construction. The housing structure 410 includes end plates412A and 412B, side plates 414A, 414B, and top and bottom ground planes414, 416. A center conductor 418 is connected between input and outputconnectors 420, 422 mounted in respective end plates 412A, 412B. In anexemplary embodiment, the connectors 420, 422 are 7 mm coaxialconnectors. The coupler 400 includes inductive and capacitive coaxialcouplers 424, 426 having respective connector ends connected to andsupported by connectors 430, 432, which are mounted in the respectiveside plates 414B, 414A. In an exemplary embodiment, the connectors 430,432 are 3.5 mm coaxial female connectors. The probe ends of the centerconductors of the probes are positioned adjacent to but not in contactwith the slab-line center conductor 418, as discussed above regardingcoupler set 150. In this exemplary embodiment, the probes are mounted inthe same plane relative to the center conductor 418, but on oppositesides of the center conductor, 180 degrees apart.

FIGS. 10A-10B illustrate another embodiment of a probe coupler mountedin a separate housing structure. FIG. 10A is a top, partiallybroken-away view of a probe coupler; FIG. 10B is an end view. The probecoupler 450 includes a separate housing structure 460, fabricated from asolid metal rectangular block, for a coaxial transmission linecomprising a center conductor 470 and an outer conductive shield surface472 formed by boring an opening in the rectangular housing block. Thehousing structure 460 includes end surfaces 462A, 462B, side surfaces464A, 464B, a top surface 468A and a bottom surface. Coaxial input andoutput connectors 480 and 482 are mounted to the respective end surfaces462A, 462B, and are connected to the coaxial center conductor 470. In anexemplary embodiment, the connectors 480, 482 may be 7 mm coaxialconnectors. A capacitive coaxial probe 490 is passed through an openingin the housing block 460 and the outer shield surface 472 so that thecapacitive probe tip is adjacent to but not in contact with the centerconductor 470. An inductive probe 492 is passed through another openingin the housing block 460 and outer shield surface 472 so that theinductive loop tip is adjacent to, but not in contact with, the centerconductor 470. The probes are electrically connected to respectivecoaxial connectors 484, 486, mounted on side surfaces 464A, 464B.

Probe couplers have been used for s-parameter measurements with a VectorNetwork Analyzer (VNA). The two coupling probes were connected to theVNA samplers as if they were the ports of distributed transmission linetype couplers that are normally used in VNA Test Sets. There are twoprinciple problems with this.

The VNA error correction math, which is well known, is based on theassumption that the signals sampled are roughly proportional to theincident and exiting (reflection or transmission) waves at each port.With probe couplers, the outputs were proportional to voltage andcurrent, not the waves. This can cause the VNA error correction math tobecome ill-conditioned, and make the error correction work poorly.

Many VNA's use the incident signal for a second purpose besidescollecting data, and that is to phase lock the RF source. The incidentsignal is used because it normally never goes to zero. However, either avoltage or current signal will go to zero or nearly so under many normalconditions. For example, when a calibration standard such as a short isconnected, the voltage or current signals will go to zero or nearly soat certain phase values. If a wide frequency range is swept, then thiscondition will almost always occur at some frequency, and maybe atmultiple frequencies.

In an exemplary embodiment, these problems may be addressed in thefollowing manner.

-   -   a) The sampled signals will be assumed to be roughly        proportional to voltage and current. After they are sampled and        detected, the signal values will be converted into values        proportional to incident and exiting waves (a and b waves for        each port) using equations 1 and 2. The new modified signal        values (a and b values for each port) can then be used in the        VNA correction math robustly. This solution will also work with        a Large Signal Network Analyzer (LSNA) such as the Maury MT6643        as well as with a VNA.        a _(n)=(v _(n) +i _(n) Z ₀)/2  equation 1        b _(n)=(v _(n) −i _(n) Z ₀)/2  equation 2

where n=port number, Z₀=characteristic impedance of the transmissionline, v_(n)=voltage at port n, i_(n)=current at port n, a_(n)=incidentwave at port n, and b_(n)=exiting wave (reflected or transmitted) atport n.

Variations of these equations may be alternatively be used. For example,often all that is needed is a value proportional to the signal, becausescaling is done later in the correction, or all that is needed areratios (such as with s-parameters). In that case, the denominator can beignored.

-   -   b) The coupling probes are still placed relatively near the DUT,        but the phase lock (or leveling) signal is taken close to the RF        source with a separate power splitter or distributed coupler        where the loss is not so critical. This would provide the more        constant signal required for that purpose. This solution will        also work with an LSNA or other instruments that require a phase        or leveling reference signal.

Subject matter described herein may address one or more of the followingproblems.

-   -   1. The coupler is very small, so is easier to mount in tight or        sensitive locations near the DUT, with or without tuners.    -   2. The coupler is very broadband, and works down to low RF        frequencies without growing in size. This is ideal for signal        analyzer or network analyzer applications, including the LSNA.    -   3. The coupler introduces very little loss, so it may be placed        between the DUT and the tuner without degrading the tuner        matching range.    -   4. Putting the coupler between the DUT and the Tuner allows:    -   a. The signal at the DUT, including the harmonics, to be        measured before the signal blockage of the tuner.    -   b. Placing the coupler between the DUT and the tuner will allow        harmonic tuning to be used with the LSNA. It will work with any        current harmonic tuning method.    -   c. The impedance seen by the DUT may be measured real-time        in-situ, meaning at the same time as the other measurements are        made. That greatly reduces or eliminates the need for time        consuming pre-characterization of system components.    -   d. Without pre-characterization, the need for high tuner        repeatability is greatly reduced.    -   e. Without the need for such high repeatability, manufacturing        costs can be reduced.    -   f. Without pre-characterization, there are fewer errors that can        occur.    -   g. Without pre-characterization, the measurement setup is much        simpler, making errors less likely.    -   h. Without pre-characterization, the RF measurement system may        be setup, and very quickly used to make DUT measurements.    -   5. The coupler device in this general application may be mounted        in a stable, fixed mounting.    -   6. The coupler in this application may be mounted inside a        tuner, eliminating the need for extra hardware blocks in the        system.    -   7. The coupler in this application may reduce costs.    -   8. The coupler may have an extended bandwidth. The coaxial probe        set 150 may have a broad bandwidth, e.g. from a few Megahertz up        to the maximum bandwidth of the transmission line into which it        is mounted. In contrast, conventional distributed couplers have        limited bandwidth based on the transmission line lengths used in        the couplers and the number of sections employed. A 1-section        coupler covers a fairly narrow band. Adding sections can        increase the bandwidth, but this also directly increases the        size and losses.

Signal coupling devices may be applied to measurement systems that usepassive tuners for reflection control by connecting the coupling devicebetween the DUT and the passive tuner.

-   -   a. The signal coupling device so applied may be any type of        device capable of signal sampling. Examples include a        directional coupler (coaxial or waveguide), a directional        bridge, a voltage-current probe, or a directional current probe.        One particular example is the probe set 150 described above.    -   b. The passive tuners may be manually controlled, or may be        automated with computer control.    -   c. The signal coupling device may be used to measure the        reflection coefficient created with a passive tuner during the        measurement, with the wave sensing device connected between the        DUT and the passive tuner.    -   d. In an exemplary embodiment, the reflection created by the        passive tuner may be measured at any desired state without        previously calibrating the tuner at multiple states.    -   e. The signal coupling device may be used to measure reflection        or transmission properties of a DUT.    -   f. The signal coupling device may be used to measure signals        coming from or going to a DUT.

A low-loss, electrically small probe, an exemplary embodiment of whichis the probe set 150, may be applied as a signal coupling device tomeasurement systems that use passive tuners for reflection control.

-   -   a. The low-loss probe so applied may be the voltage-current        probe set 150. It also may be configured or constructed in a        variety of other ways, including but not limited to a pair of        voltage probes with a phase offset between them, or a pair of        current probes with a phase offset between them. Note: when two        similar probes are used, the ideal separation along the tuner        center conductor would 90 degrees of transmission phase (quarter        wave), but may be usable in many applications over some range. A        rule of thumb for the usable range would be a transmission phase        over the range of about 10 degrees to 170 degrees.    -   b. The low-loss probe may be built into the passive tuner        housing.    -   c. The probe may be built into a separate housing and connected        to the tuner. The separate housing does not need to be connected        directly to the tuner, but may be connected through coaxial        cables, waveguide, or other devices capable of transmitting the        signal.    -   d. If a 2-port tuner is used in the measurement system, the        low-loss probe may be placed on the side of the tuner near the        DUT, or on the side of the tuner away from the DUT.    -   e. If a 1-port tuner is used in the measurement system, the        low-loss probe may be connected between the DUT and the tuner.    -   f. If the low-loss probe is connected between the DUT and the        tuner, the reflection created by the tuner at any desired state        may be measured without pre-calibrating the tuner at multiple        states.    -   g. The passive tuners may be manually controlled, or may be        automated with computer control.    -   h. The low-loss probe may be used to measure reflection or        transmission properties of a DUT.    -   i. The low-loss probe may be used to measure signals coming from        or going to a DUT.

A fixed housing in which a low-loss, electrically small probe ispermanently mounted provides advantages:

-   -   a. The fixed housing eliminates the problem with repeatability        of the probe height over the main transmission line.    -   b. The fixed housing eliminates the problem with measurement        repeatability due to misplacement of the probe along the        transmission line.    -   c. The fixed housing eliminates the problem with coupling        repeatability due to misalignment of the probe on the        transmission line.    -   d. The fixed housing eliminates coupling variability due to        variation of the main transmission line dimensions or        characteristic impedance.    -   e. The fixed housing eliminates stability problems during the        calibration process, while multiple standards are connected to        the circuit.

The bandwidth of a voltage-current probe may be improved by mounting thesensors so the reference plane of the current and voltage sensors areco-planar. Regardless of the orientation of the probe pair, preferablythe loop of the inductive probe is parallel to the center conductortrace.

The data correction algorithm used with a voltage-current probe may bedesigned to eliminate ill-conditioned data when either the voltage orcurrent on the main transmission line becomes small at the point wherethe probe is sensing the signal.

-   -   a. The voltage-current probe has two sensors, each providing a        signal to be measured. The new algorithm (described above        regarding equations 1 and 2) treats these two sensor signals as        being proportional to voltage and current on the main        transmission line in the error correction math calculations.    -   b. One implementation of the math calculation is to convert the        measured values of the two sensor signals from voltage-current        signals to incident-reflected waves using equations 1 and 2.

The resulting a and b values are then used in the error correctioncommonly used in Vector Network Analyzer (VNA) measurements.

-   -   c. The math implementation may be done in different ways,        depending on how the equations are combined or manipulated with        the equations with VNA error correction equations.    -   d. Although error correction in a VNA is used here as an        example, this improved algorithm is not limited only to a VNA.        It may be used with any measurement system that samples a signal        from a main transmission line.    -   e. Conventionally, the measured values of the two sensor signals        were used directly in the error correction used in a VNA.        Although it can be corrected in principle by the VNA error        correction routine, the data could become ill-conditioned when        the average voltage or current value on the main transmission        line became very small at the reference plane of the probe. The        ill-conditioned data causes the calculation to be very sensitive        to small errors, and the residual error after correction may        become significant.

In accordance with one embodiment, signal coupling devices may beapplied to load pull systems that use passive tuners for reflectioncontrol by connecting the coupling device between the DUT and thepassive tuner.

-   -   a. In general, a load pull system is one where impedance is        controlled or varied, and a set of parameters is measured for        each measurement condition. The impedance control is typically        done with impedance tuners of some type. There are many types of        passive impedance tuners, including but not limited to        slide-screw tuners, double slug tuners, stub tuners, and solid        state tuners using PIN diodes or other solid state control        devices. The passive tuners may be manually controlled, or may        be automated with computer control.    -   b. The signal coupling device so applied may be any type of        device capable of signal sampling. Examples include a        directional coupler (coaxial or waveguide), a directional        bridge, a voltage-current probe, or a directional current probe,        including the probe set 150 described above.    -   c. The signal coupling device is used to sample signals between        the DUT and passive tuner, in order to measure the signal        applied to or coming out of the DUT as required by the load pull        application, and to measure the reflection coefficient created        with a passive tuner during the measurement.    -   d. The reflection created by the passive tuner may be measured        at any desired state without previously calibrating the tuner at        multiple states.    -   e. If the tuner was previously calibrated, the reflection        created by the passive tuner can be measured at any desired        state and compared to the calibrated data as a verification that        the system is working correctly, and also to give an indication        of system measurement accuracy. The pre-calibration data may        also be used to determine tuning settings prior to the actual        measurement with the DUT.    -   f. The signal coupling device may be used to measure reflection        or transmission properties of a Device Under Test (DUT), by        measuring incident and reflected or transmitted signals, and        applying the appropriate math, such as the complex ratio of the        output signal to the input signal, for example.

In accordance with another embodiment, a low-loss, electrically smallprobe may be applied to load pull systems that use any type of impedancetuning for reflection control by connecting the coupling device betweenthe DUT and the controlled impedance.

-   -   a. This applies to measurement systems that use passive        impedance tuning or active impedance tuning. As noted above,        there are many types of passive impedance tuners, including but        not limited to slide-screw tuners, double slug tuners, stub        tuners, and solid state tuners using PIN diodes or other solid        state control devices. Active impedance tuning may take any form        where the reflection termination seen by the DUT is at least        partially synthesized by injecting a signal back to the DUT. The        injected signal may be created in different ways, such as from a        separate source or by coupling off part of the input or output        signal and feeding it back to the DUT with controlled magnitude        and phase.    -   b. In general, a load pull system is one where impedance is        controlled or varied, and a set of parameters is measured for        each measurement condition. The impedance control is typically        done with impedance control of some type. The tuning may be        manually controlled, or may be automated with computer control.    -   c. The low-loss probe so applied may be the probe pair or set        150 described above. It also may be configured or constructed in        a variety of other ways, including but not limited to a pair of        voltage probes with a phase offset between them, or a pair of        current probes with a phase offset between them. Note: when two        similar probes are used, the ideal separation would 90 degrees        of transmission phase (quarter wave), but may be usable in many        applications over some range. A rule of thumb for the usable        range would be a transmission phase over the range of about 10        degrees to 170 degrees.    -   d. The signal coupling device is used to sample signals between        the DUT and tuning mechanism, in order to measure the signal        applied to or coming out of the DUT as required by the load pull        application, and to measure the reflection coefficient created        by the tuner during the measurement.    -   e. If a 2-port tuner is used in the measurement system, the low        loss signal coupling device may alternately be placed on the        side of the tuner away from the DUT. In this case, the        de-embedding of the measured data through the tuner to the DUT        reference plane may be performed using methods currently used in        traditional load pull setups.    -   f. The reflection created by the tuning may be measured at any        desired state without previously calibrating the tuner at        multiple states.    -   g. If the tuner was previously calibrated, the reflection        created by the tuner can be measured at any desired state and        compared to the calibrated data as a verification that the        system is working correctly, and also to give an indication of        system measurement accuracy. The pre-calibration data may also        be used to determine tuning settings prior to the actual        measurement with the DUT.    -   h. The signal coupling device may be used to measure reflection        or transmission properties of a DUT, by measuring incident and        reflected or transmitted signals, and applying the appropriate        math, such as the complex ratio of the output signal to the        input signal, for example.    -   i. The low-loss probe can be built into the housing of the        tuning mechanism.    -   j. Alternatively, the probe can be built into a separate housing        and connected to the tuner. The separate housing does not need        to be connected directly to the tuner, but may be connected        through coaxial cables, waveguide, or other devices capable of        transmitting the signal.

In accordance with a further embodiment, a low-loss, electrically smallprobe may be applied to load pull systems that use passive tuners forreflection control by connecting the coupling device between the DUT andthe passive tuner.

-   -   a. In general, a load pull system is one where impedance is        controlled or varied, and a set of parameters is measured for        each measurement condition. The impedance control is typically        done with impedance tuners of some type. There are many types of        passive impedance tuners, including but not limited to        slide-screw tuners, double slug tuners, stub tuners, and solid        state tuners using PIN diodes or other solid state control        devices. The passive tuners may be manually controlled, or may        be automated with computer control.    -   b. The low-loss probe so applied may be the voltage-current        probe set 150. It also may be configured or constructed in a        variety of other ways, including but not limited to a pair of        voltage probes with a phase offset between them, or a pair of        current probes with a phase offset between them. Note: when two        identical probes are used, the ideal separation would 90 degrees        of transmission phase (quarter wave), but may be usable in many        applications over some range. A rule of thumb for the usable        range would be a transmission phase over the range of about 10        degrees to 170 degrees.    -   c. The signal coupling device is used to sample signals between        the DUT and passive tuner, in order to measure the signal        applied to or coming out of the DUT as required by the load pull        application, and to measure the reflection coefficient created        with a passive tuner during the measurement.    -   d. If a 2-port tuner is used in the measurement system, the low        loss signal coupling device may alternately be placed on the        side of the tuner away from the DUT. In this case, the        de-embedding of the measured data through the tuner to the DUT        reference plane using methods currently used in traditional load        pull setups.    -   e. The reflection created by the passive tuner may be measured        at any desired state without previously calibrating the tuner at        multiple states.    -   f. If the tuner was previously calibrated, the reflection        created by the passive tuner may be measured at any desired        state and compared to the calibrated data as a verification that        the system is working correctly, and also to give an indication        of system measurement accuracy. The pre-calibration data may        also be used to determine tuning settings prior to the actual        measurement with the DUT.    -   g. The signal coupling device may be used to measure reflection        or transmission properties of a Device Under Test (DUT), by        measuring incident and reflected or transmitted signals, and        applying the appropriate math, such as the complex ratio of the        output signal to the input signal, for example.    -   h. The low-loss probe can be built into the passive tuner        housing.    -   i. The probe may be built into a separate housing and connected        to the tuner. The separate housing does not need to be connected        directly to the tuner, but may be connected through coaxial        cables, waveguide, or other devices capable of transmitting the        signal.

Conventional pre-characterization typically means to measure theproperties of the tuner (without limitation but typically by measuringthe s-parameters of the tuner) at multiple tuning settings, and savingthe data for all characterized settings in a file. In accordance withanother embodiment, that pre-characterization will still work, althoughsimpler and potentially less accurate methods may also be employed,since final accuracy will be determined at measurement time, not by thepre-characterization. In accordance with another embodiment, anexemplary algorithm to select tuner settings using a signal coupler in aload pull measurement system is illustrated in flow diagram form asalgorithm or method 500 in FIG. 11. At 502, the tuning mechanism, e.g.an impedance controlling tuner, is pre-characterized at a number ofimpedance points over a desired tuning range. This step is similar tothe techniques commonly employed to pre-characterize tuners inconventional load pull systems, except that less accuracy may beobtained, since during the actual measurement, the actual reflectioncoefficient will be measured. And because less accuracy is obtained, thedesired impedance setting may be obtained from fewer characterizepositions, resulting in more rapid tuner calibration orpre-characterization.

At 504, the pre-characterized tuning mechanism is set-up in the loadpull measurement system, with the signal coupling device in a signalpath between the DUT and the tuning mechanism. It is to be understoodthat the steps 502 and 504 may be reversed in order, in that the tuningmechanism may be installed in the measurement system andpre-characterized in situ. At 506, a tuning setting for the tuningmechanism is determined in order to achieve a particular desired ortarget reflection termination for the DUT, e.g. by selecting one of thepre-characterized tuning points or by interpolating between thepre-characterized impedance tuning points, and the tuning mechanism isadjusted to this tuning setting at 508. The actual reflectioncoefficient seen by the DUT is measured, using the signal couplingdevice between the DUT and the tuning mechanism (510). If at 512, 514,the difference (error) between the target reflection coefficient and themeasured coefficient is larger than desired for the particularapplication, then a new tuner setting is calculated at 516, offset bythe error from the desired impedance. Steps 508 and 510 are repeated,and the process iterated. At 518, the load pull parameters are measured,and the process repeats if additional impedances are to be measured forthe desired measurement set. Of course, other embodiments of exemplaryalgorithms may include a different number of loops or nested loopsemployed to control multiple impedance variables or other independentvariables, e.g. in a manner similar to sweep plan loops employed inconventional load pull systems,

An exemplary algorithm may include the following:

-   -   a. This algorithm applies, in an exemplary embodiment, to a load        pull system where the coupling device is connected in a way to        allow measurement of the reflection coefficient presented to the        DUT with the DUT in place. One implementation of this is to        connect the signal coupling device between the DUT and the        tunable impedance.    -   b. The tuner mechanism may be pre-characterized over some        desired tuning range in a manner similar to pre-characterizing        tuners in current, traditional load pull systems, except for the        following:    -   1) Less accuracy may be required, because during the actual        measurement, the actual reflection will be measured. Only an        approximate reflection coefficient is normally required for this        impedance setting algorithm.    -   2) Because less accuracy may be required, the desired impedance        setting can be interpolated from fewer characterized positions.        Fewer characterized points result in a quicker tuner        calibration.    -   c. At measurement time, the impedance tuning setting may be        determined by selecting one of the calibrated impedance points,        or by interpolating between the calibrated impedance tuning        points, to achieve a particular desired target impedance        presented to the DUT. The advantage of this is that one will not        normally have to hunt for the desired impedance at measurement        time, except for fine tuning in special cases. Note: the process        of actually interpolating between calibrated impedance points is        already implemented in commercial load pull systems.    -   d. After the tuning setting is determined and set, the actual        impedance achieved by that setting will be measured. In most        load pull measurement situations, the exact impedance is not        critical as long as it is known, so the system is then ready to        measure the normal load pull parameters.    -   e. If the exact impedance setting is critical and the actual        measured impedance is not close enough, a new impedance offset        by the error from the desired impedance may be determined and        set, and the actual impedance re-measured. This process can be        iterated until a sufficient accuracy is achieved.

One embodiment of a simpler characterization is to characterize a tunerover its specified frequency and tuning range, and use curve fittingtechniques to mathematically model that tuner vs. Tuning position andvs. Frequency. The initial characterization may be done as discretepoints, and the mathematical model would allow interpolated data for anypoint.

Another embodiment of a simpler characterization is to develop a generalcharacterization for all tuners of a specific model number. This may beaccurate only to the unit to unit repeatability in the manufacturingprocess.

Although the foregoing has been a description and illustration ofspecific embodiments of the subject matter, various modifications andchanges thereto can be made by persons skilled in the art withoutdeparting from the scope and spirit of the invention.

What is claimed is:
 1. A measurement system for conducting measurementson a device-under-test (DUT), wherein impedance is controlled or variedover a set of measurement conditions and a parameter or set ofparameters measured for each measurement condition, the systemcomprising: a passive impedance controlling tuner; a signal transmissionline, the passive impedance controlling tuner including a signaltransmission line segment as at least part of the signal transmissionline; a signal coupling device coupled in a non-contacting relationshipto the signal transmission line between a signal port of the DUT and thepassive impedance controlling tuner for sampling signals propagatingbetween the passive impedance controlling tuner and the DUT to allowmeasurement of an actual impedance presented to the DUT with the DUT inplace in the measurement system during measurement of DUTcharacteristics; measurement system equipment for receiving responsesignals from the signal coupler; and wherein the measurement system isconfigured to conduct said measurement of DUT characteristics withoutpre-characterizing said passive impedance controlling tuner.
 2. Thesystem of claim 1, wherein the signal coupling device is mounted in afixed position relative to the DUT.
 3. The system of claim 1 wherein thesignal coupling device is a separate device relative to the passiveimpedance controlling tuner.
 4. The system of claim 1, wherein thesignal coupling device is integrated with the passive impedancecontrolling tuner.
 5. The system of claim 1, wherein the DUT is selectedfrom the group comprising power transistors, power FETs or other poweramplifying devices, small-signal low noise transistors, amplifyingdevices, frequency translating devices, frequency multipliers,two-terminal devices, three-terminal devices, and mixers.
 6. The systemof claim 1, wherein the passive impedance controlling tuner is selectedfrom the group consisting of slide-screw tuners, double slug tuners,stub tuners and solid state tuners.
 7. The system of claim 1, whereinthe passive impedance controlling tuner is manually controlled.
 8. Thesystem of claim 1, wherein the passive impedance controlling tuner isautomated with computer control.
 9. The system of claim 1, wherein thesignal coupling device is selected from the group consisting of adirectional coupler, a directional bridge, a voltage-current probe, or adirectional current probe.
 10. The system of claim 1, wherein said DUTcharacteristics comprises a set of load pull parameters.
 11. The systemof claim 1, wherein the signal coupling device is adapted to allowmeasurement of phases and amplitudes of said sampled signals.
 12. Amethod for using a measurement system including a passiveimpedance-controlling tuner and a signal transmission line, the tunerincluding a signal transmission line segment as at least part of thesignal transmission line, the method comprising a sequence of thefollowing steps: coupling a signal coupling device in a non-contactingrelationship to the signal transmission line of the measurement systemto allow measurement of an impedance presented to a device-under-test(DUT) with the DUT in place; for DUT measurements, withoutpre-characterizing the tuner, setting the impedance tuning to a setting;using the signal coupling device to measure an actual impedancepresented to the DUT after the impedance tuning setting is set; andconducting measurements of the DUT using the measurement system.
 13. Themethod of claim 12, wherein said setting the impedance tuning furthercomprises: setting the tuner to a new tuner impedance determined by anoffset between a target impedance and the measured actual impedance;measuring an actual impedance presented to the DUT at the new tunerimpedance setting; and iterating said steps of setting the tuner to anew tuner impedance and measuring the actual impedance until a desiredimpedance accuracy is achieved.
 14. The method of claim 12, wherein saidconducting measurements of the DUT comprises: measuring a set of loadpull parameters.